There are many potential reasons behind the yellowing of your plant's leaves, however, here are some of the most common reasons:
Lack of sunlight
Plants rely on how much sun they get each day, and if they're facing a lack of sunlight, this may be quite a big part of the problem. If you know you're giving the plant enough water, and the soil does not feel dry when you check it, then it may be time to move your plant around- try placing it closer to the window, or into a room that gets much more sunlight than others.
Dehydration vs. Waterlogging
The most common reason for leaves turning yellow is because of too little or too much water. However, as easy as it's to say that's the problem, it can be rather tricky to understand whether you're overwatering or underwatering your plant. One little trick in telling the difference is by looking closely at the leaves. If they're drooping slightly and seem rather limp, this is likely to be a sign of overwatering. Contrarily, if they're curling up a little and seem crispy, you've probably not been watering your plant enough.
However, sometimes it's not about how much you're watering, and instead the water is not getting to the roots effectively. This can then lead to us gardeners presuming we're not giving them enough water, and then watering so much it ends up becoming waterlogged. In this case, we recommend improving the plant's soil drainage by moving the plant to a raised bed or mixing in some sand into the soil.
If the leaves on your plants are not only yellow, but have holes/parts of the leaves munched off, it's more than likely that you have a pest eating away at your plant.
One way to get rid of these pests immediately, without damaging the plant, is by using insecticidal soap.
It's none of the above; why are my leaves still turning yellow?
If you've already crossed off all the above, and everything you try doesn't seem to make any difference, it's probably time to dig a bit deeper and look closer at the leaves.
Often, leaves will turn yellow because of some fungus or disease they have become a victim of. Some of the most common diseases include leaf spot or blight. Leaf spot is rather contagious and will spread rapidly- if you notice the disease early on, dispose of any infected leaves immediately, and continue to do so. The plant will eventually recover naturally.
Another very common problem is the possibility that the plant is missing some nutrients. The three most important nutrients every plant needs is nitrogen, phosphorus, and potassium. Secondary nutrients include magnesium, sulphur, and calcium. Many of these nutrients can be found in composts or fertilisers which you can then apply to the soil of your plants. We also recommend adding organic matter, like grass clippings, to the soil- these also include some of the other nutrients plants require in small amounts (i.e., copper/iron/boron/zinc etc.).
With that being said, the best thing you can do is to get your soil tested for any missing nutrients- and if there any missing, ensure you mix in some compost or fertiliser etc. that will help balance these nutrients out.